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A mach number is a number that describes speed. It is a measure of the speed of an object compared to the speed of sound. Mach number is given as a percentage compared to the local speed of sound. For example, half the speed of sound = Mach 0.5.
Where v is the speed of the object, and a is the speed of sound.
It is named after the Austrian physicist Ernst Mach. When writing a speed as a Mach number, the number comes after "Mach." For example, the Concorde airplane could fly at Mach 2, and a satellite re-enters the Earth's atmosphere at about Mach 25.